The thermal treatment of pellets, in particular the fire-hardening of iron ore pellets, mostly is effected on travelling grates with gas hoods, which are referred to as indurating machine. As seen in running direction, the pellet firing machines have various, possibly further sub-divided treatment zones, in particular a drying zone, thermal treatment zones for preheating and firing, and a cooling zone. The required process heat is generated by combustion of liquid, gaseous or solid fuel. To optimize the energy utilization, gas recirculation systems are provided.
From EP 0 030 396 B1, for example, a method for the thermal treatment of pellets is known, in which the unfired pellets are conveyed over a travelling grate and dried in a pressure drying zone and a suction drying zone by means of recirculated process gases. In a heating zone and a firing zone, heated cooling gases are sucked through the pellet layer. Said gases are supplied from the cooling zone via a recuperation conduit and lateral supply ducts to 38 firing chambers distributed along the length of the firing zone, heated there with 38 oil burners and via firing chamber outlets supplied to the heating and firing zone, in which in addition solid fuel provided on the surface of the pellet bed is burnt. In dependence on the fuel used and the burner capacity, very high flame temperatures can occur, which leads to a stress of the refractory material and increases the nitrogen oxide (NOx) emissions. Since the air supplied to the firing chambers via the supply ducts impinges on the firing flame from above at an angle of 90°, said flame is deflected and contacts the refractory-lined wall of the firing chamber, which can lead to damage. The impulse of the cold primary air here is too low to generate a flame-stabilizing spin. On the other hand, the amount of primary air cannot be increased without an undesired increase of the fuel consumption. In addition, considerable heat losses occur at the walls of the lateral supply ducts for the firing chambers, due to the large surface area.